5 Answers
5

When you write in active voice, it means that you're using a standard [subject] [verb] [object] sentence.

Jack killed Mary.

When you use the passive voice, it means that you reverse the subject and the object from the active form.

Mary was killed by Jack.

The meaning of the sentence is the same: Jack is doing some killing. Yet Jack is not the subject and killing is not the main verb. The sentence emphasizes all the wrong parts of itself by being put in this order. It uses more words and the picture it gives isn't as clear. When an action is performed, make sure you use that word as the action verb, and you will do just fine.

I just noticed some fun irony in "When an action is performed" while I was re-reading :P
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StrixVariaDec 9 '10 at 1:46

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+1: Professional writing often stresses "punch" and active voice has much more than the wishy-washy passive voice.
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SatanicpuppyDec 9 '10 at 19:04

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The sentence emphasizes all the wrong parts of itself by being put in this order - Well, it depends. If earlier the police stated that Jill killed Mary, but then later Hercules Porrott turns up and figure out what really happened...
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Charles StewartJan 17 '11 at 11:51

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Essentially, be aware of the voice you are using and you will be able to convey subtle shades of meaning and tone by switching appropriately. Think about how each sentence sounds in either voice and you'll get into a habit of doing this without conscious thought. An interesting note: everyday speech is much heavier in the passive voice than descriptive writing should be. If you write exactly as you would speak (and you're not writing dialogue) you probably are writing in the passive voice.
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atroonJan 17 '11 at 17:01

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@Satanicpuppy I really wish we could put to death the notion that the passive voice is "wishy-washy" or "weak". Sometimes the passive voice is exactly what you need, and generations of newbie writers have been led astray by the false idea that you should avoid the passive for some reason.
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JSBձոգչNov 19 '12 at 0:43

An active sentence means that the subject is the one performing the action, that he is the one being active in the sentence. A passive sentence is a sentence where the action is being performed onto the subject, who is passively enduring the action being done upon it. If you have a sentence for which you can not tell if it is passive or active, just ask yourself if the subject's the one doing the action or is the action being done to the subject.

Since this is a site for writers, I assume you are more interested in the practical use of the passive and active voices than it's grammatical explanation. A lot of people will tell you that using passive voice is bad. Here, I must stand in defence of the passive voice. Using passive voice is not by definition bad. Passive has its uses and its purpose. It is like saying antibiotics are bad. Sure, if you use antibiotics for every little soar throat, you're not doing yourself any favours, but in certain conditions, they are the only thing that can help you. The same goes for passive voice. The trick is to know when to use it.

Here is the example where a passive voice is considered "weak":

The battle was lost. The city was ransacked, the men slaughtered and women raped or sold to slavery.

Firstly, my apologies to the women in the example. If you look at the sentences, you will notice that they're not exactly bursting with action. Yes, horrible things are happening, but it doesn't feel energetic. That's because all the action is being done to a passive subject. If we change it to:

They won the battle. They ransacked the city, slaughtered the men and raped the women or sold them to slavery.

This example is much more energetic. It's because the subject is actively participating in the action. It is causing it to happen. This is where the scene profits from the use of active voice, and this is the reason they say passive is "bad". If you need a dynamic, active scene, use active voice when ever you can.

But this does not mean passive really is bad. Passive has its uses. The obvious ones being when we don't know or care who is doing the action, or it's implied:

It has already been done.

The Cathedral was built in sixteen century.

It is usually implied who builds cathedrals - builders commissioned by the Church. Or we simply don't care, we are interested in the cathedral and when it was built, not in those who built it.

The less obvious, more subtle uses of passive are when you don't want the action to be dynamic. Perhaps you want to create a feeling of inaction, of subject's helplessness or unwillingness to take the action.

As he was being beaten, the only thing he could do was to curse his assailants in his mind.

Too drunk to stand, he was carried off to his room.

Sure, They carried him off to his room is more dynamic, more active, but it's not what we want here. We want to show the subject's inability to perform the action, to emphasize his passiveness.

The sentence "The city was ransacked, the men slaughtered and women raped or sold to slavery." implies a certain helplessness of the city and it's people to defend themselves from the fate that has befallen them. Maybe that's more important to us in that moment than the action. It can also depend on whose side we are, the winning side or the loosing side. So, the passive has it's uses. A skilled writer knows when to turn a "weakness" into an advantage.

Scientists and academics often use--rightly--the passive voice to shift focus from the actor to what is being acted upon. For example, "dry ice was dropped into the at and the ensuing results recorded." The actor is unimportant, so passive voice is acceptable.

In answer to Charles Stewart:
I would suggest that any verb with a subject must be active or passive. Those are the only constructions we have. We can make passive versions of all your examples, so they must by definition be active voice.

"There were a great number of dead leaves lying on the ground." There is no modal here; we have the existential "there" construction with "be" (not a modal) , where "There" is a dummy subject delaying the 'real' subject "dead leaves". This sentence is active voice - Compare with the passive - "There are said to be a great number of leaves lying on the ground".

"If six was nine" - we definitely have a subject, and it uses a normal active construction, so this is also active voice. Compare the passive - "If six was added to nine ..."

Infinitives can also be subjects in active constructions - "To have loved and lost is better than not to have loved at all." Compare the passive - "To have been loved ...". Your Startrek sentence fragment has no verb, so is obviously neither active nor passive voice. But we can even give that a passive infinitive - "To be boldly led where no man has gone before!" - infinitives have two passive forms - "to be done" and "to have been done" - so the standard forms are active.

I would argue that standard imperatives are also active as you can also have passive imperatives - "Please be assured", and the subject is understood.

You say that active is better than passive "because you don't need an auxiliary verb". This is only true in present and past simple positive forms. In EFL/ESL we use a twelve tense system. If we take it that each tense has three forms: positive, negative and interrogative, that means that only in two out of thirty-six forms is this true.

Active "is almost always simpler and briefer". But in some 80% of passive sentences we don't use an agent, so this doesn't necessarily follow at all. "They sacked him." "He was sacked." And You can be just as formal and wordy in the active as in the passive.

A few other occasions when it is appropriate to use the passive -
a. When the agent is unknown, important or obvious - "He's been arrested."
b. To avoid a long subject - "We were very impressed by his obvious enthusiasm for the subject." (end-weighting)
c. To start a sentence with familiar 'given' information, and introduce new information later in the sentence - "They just arrived five minutes ago. Their train was delayed because of accident."

@Jonny - The easy answer to your question is that a verb is active if it is in a normal tense form and is not in a passive construction, which involves different forms of the verb 'to be' + the past participle of the main verb.

He writes books / was writing a book / will have written a book - active
This book was written a long time ago/ is being written right now/ had been written in Latin first - passive

I'm not a native English speaker, and probably not aware of all the grammatical properties of "There was" sentence structure, but shouldn't the passive of "There were a great number of dead leaves lying on the ground." be "There were a great number of dead leaves laid on the ground."? Dead leaves being the real subject?
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TannaleinNov 20 '12 at 7:56

@kindall: "Laid implies that someone put them there" - isn't that the very definition of passive? That someone else did the action? Would the form be any more active if "There were people lying on the ground"? I don't think a subject in a sentence is required to be capable of moving on its own accord.
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TannaleinNov 26 '12 at 11:41

The question I was answering was whether laid or lying was correct. Oh, sorry, I see that you were using "laid" not "were laid" -- I read your previous comment too quickly. You're correct, and I deleted my previous comment.
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kindallNov 26 '12 at 16:58

A verb is in the active voice if the agent of the verb (i.e., the pronoun/noun phrase that does the verb) is in the subject position (usually, but not always before the verb); it is in passive voice if the agent is in object position. Not all verbs are in either the active or the passive voice.

Examples

Geoffrey Pullum has three examples of sentences wrongly characterised in Strunk&White as passive in his 50 years of stupid grammar advice anniversary polemic, the first of which is:

There were a great number of dead leaves lying on the ground.

Here there are two verbs, the first of which has no agent (it is a modal verb), the second of which is a participle, with its agent in subject position.

Not all verbs are in either the active or passive voice:

If six was nine. — "was" is here, I think, most widely considered to be not an agentive verb but a stative verb, so this sentence has neither agent nor patient, instead it has a stative subject and a predicative object.

To boldly go where no man has gone before! — Infinitives are agentless;

Keep it to yourself! — The agent of a command is presumed to be whomever the command is addressed to, and need not be specified.

How to choose

The active voice has a built-in advantage that it is almost always simpler and briefer, because you don't need an auxiliary verb, which passive constructions need in English. So use the active unless there is a compelling reason not to.

The two most important reasons why the passive might be appropriate are, first, it can give finer control over information conveyed (e.g., Mary was killed!) or secondly, it can change emphasis by giving control over the order in which information is presented (e.g., It was Mary who Jack killed.)